Moulding of superplastic alloy sheet

ABSTRACT

A superplastic alloy sheet material which has been removed from a mould is held at a temperature of from 180* to 220*C to remove undesired warping or distortions from the moulded sheet.

llmted States Patent [111 3,864,176 Swanson Feb. 4, 1975 MOULDING OF SUPERPLASTIC ALLOY SHEET [56] References Cited [75] Inventor: Colin John Swanson, Horfield, U I ED S ATES PATENTS England 3,567,524 3/197] Cook ct all 148/! [.5 R 3.595060 7197i H d I48 ll.5 R [73] Asslgnee? ISC Alloys London 3 676 115 71972 H51 1: I. 1424;] 1.5 R

England [22] Filed: June 14, 1973 Primary Examiner-W. Stallard pp No: 370,035 Attorney, Agent. or [vrmH0lman & Stern [57 ABSTRACT [30] Fomgn Apphcamn Data A superplastic alloy sheet material which has been re- June 14, Great Britain moved from a mould is at a temperature of from l80 to 220C to remove undesired warping 0r distor- U.S. v i 1.5 R [ions from [he moulded sheeL 1] Int. Cl. C2ld 9/48, C22f 1/04 5 Field of Search 148/] 1.5 R

9 Claims, N0 Drawings MOULDING OF SUPERPLASTIC ALLOY SHEET This invention relates to moulding of superplastic alloy sheet material.

The composition and properties of some superplastic alloys are described in detail in the article Superplasticity in an Al-Zn Alloy by BackofemTurner and Avery, Transactions of the A.S.M., Volume 57 (1964), pages 980-990.

An important property of superplastic alloys is that their strength under certain conditions is extremely low; thus when fabricating superplastic alloys deformation forces will be small. It has been demonstrated that superplastic alloy sheets may be thermoformed in a manner similar to conventional plastics materials; a typical group of superplastic alloys which may be thermoformed are those based upon the Zinc-Aluminium eutectoid composition, viz 78 percent Zn 22 percent Al.

It will be appreciated that for the moulding process to be commercially viable a formed component should be removed from the die as soon as possible after forming. Thus whilst on the one hand the low strength of the alloy is advantageous during the forming operation, on the other hand this property may result in distortion of the alloy sheet when attempting to remove the component from the die at an elevated temperature.

Various methods may be employed to overcome this difficulty. For example, the formed component may be heated for several minutes in the die from the forming temperature of 250 to 270C to a temperature in excess of 275C, for example 300C, and the component then removed. This method relies upon the fact that the superplastic structure of a Zn-Al eutectoid alloy is only stable up to 275C, above which temperature the metallurgical structure is irreversibly transformed into a non-superplastic structure. This means that the structure of the alloy sheet is stronger above 275C than below. To avoid the necessity of cycling the die temperature, the die is maintained at 300C while the sheet to be thermoformed is preheated to 250C. When, during forming, the sheet impinges on the die its temperature is raised locally above 275C and transformation of the structure occurs. It is essential that the component should be formed as quickly as possible otherwise transformation might occur in portions of the sheet before they impinge on the die. This method suffers from various disadvantages, in particular the transformation from one metallurgical structure to another takes a finite time with the probable result that, in a complicated component, there will be micro-structural variations which could lead to variations in the mechanical properties of the component. A further disadvantage of this method is that temperature variations in the unformed sheet are likely to occur. When in the superplastic state the hotter areas of the sheet will be weaker during deformation and excessive thinning and even perforation of the sheet may occur.

A variation of the method described above would be to employ a cold die so that on striking the die the sheet cools and is thus strengthened. This method suffers from the disadvantages that only components which require short forming times are amenable to this technique and thus the amount of detail and sharpness of angles which may be moulded are restricted. Also local thinning may be accentuated due to temperature variatrons.

It may be considered suitable to cool both the component and the die to a temperature at which the component is sufficiently rigid to withstand distortion during removal from the die. Such a technique, however, results in unacceptably long forming cycles. Alternatively, forced cooling of the component could be employed wherein heat is absorbed from the component at a faster rate than the component absorbs heat from the die. it will be appreciated that various methods may be employed to achieve a significant drop in the component temperature for only a small decrease in the die temperature. However, forced cooling of one side of a component will give rise to temperature gradients and variations in degree of thermal contraction tending to distort the component. Thus although the formed article may be sufficiently cooled and thus strengthened to withstand distortion during removal from the die the cooling process itself will tend to cause distortion.

The invention provides a method of moulding superplastic alloy sheet material comprising holding a moulded sheet, after the latter has been removed from a mould, at a temperature of to 220C to remove undesired distortions or warping from the moulded sheet.

The invention consists in a method of moulding superplastic alloy sheet material comprising heating a blank of the sheet material to a temperature at which the material exhibits superplasticity, moulding the heated blank to conform to a mould surface of desired shape. removing the moulded sheet from the mould, and holding the moulded sheet at a temperature of from 180 to 220C to remove undesired warping or distortions from the moulded sheet.

The heat treatment of the moulded sheet is preferably carried out for at least 10 minutes, and may be done in a suitable oven. Alternatively, the heat remaining in the moulded sheet after the moulding operation may be utilised for removing the distortions. In such a case the hot moulding is supported in a mould of complementary shape and allowed to cool ensuring that it is held for at least 10 minutes at from 180 to 220C.

Either surface of the moulded sheet may be cooled after moulding, i.e., either the surface adjacent the mould or the surface remote from the mould may be cooled to facilitate removal of the moulded sheet from the mould surface. The cooling of the surface of the sheet may be effected by air blowing or water spraying.

During heating the sheet should preferably be supported on a surface which is generally complementary to the overall shape of the sheet, that is a flat surface is employed for a flat sheet and a surface of suitable curvature for a curved sheet.

The heated blank may be subjected to a moulding operation in which it is caused to conform to a mould surface of desired shape by means of an applied fluid pressure or vacuum.

The method according to the invention is most suitably applied to large flat components with small depths of draw when the component only requires to be supported in a horizontal position on a flat surface. With deep draws and for more complex shapes the components will require to be supported by an accurately contoured jig of a material sufficiently rigid to withstand the annealing temperature.

The sheet material may suitably be a Zinc- Aluminium alloy containing from 15 to 40 percent, preferably 18 to 30 percent, by weight of aluminium,

with or without minor additions (up to 0.5 percent by weight) of a third component, particularly copper or magnesium.

Various methods may be used for rendering such materials superplastic but in general include the steps of a. Heating the body of alloy to homogenize it.

b. Quenching or slow cooling the homogenzied alloy.

c. Working the alloy. preferably by rolling.

Suitable procedures are described in British Patent Specification Nos. l,l 25,072 and l.225,8 l9. Slow cooling may be substituted for quenching under appropriate conditions, especially for alloys containing l8 to 40 percent by weight of aluminium.

Although the method according to the invention as described is particularly applicable to alloys of zinc containing between and 40 percent by weight of aluminium the method could be similarly applied to other superplastic metals.

The invention will be further described with reference to the following examples:

EXAMPLE 1 A rectangular wall panel of dimensions 1m X /zm and a depth of draw less than 3cm was pressure moulded in superplastic zinc/aluminium alloy sheet containing 78 percent by weight of zinc and 22 percent by weight of aluminium of thickness 0.035 inches for 12 minutes and at 250C, using an air pressure of 60 lbs per square inch.

Due to the complexity of the design detail in the panel it was impossible to remove the moulded panel from the mould surface without distorting it.

The distorted moulding was stored and at a convenient time was placed on a flat hot plate at a temperature of 200C. The panel was freed from distortions within about 15 to minutes.

EXAMPLE 2 A moulding of overall dimensions 30 inches X 13 inches X 8 inches deep having a central dome and a peripheral flange was pressure formed in superplastic zinc/aluminium alloy sheet containing 78 percent by weight of zinc and 22 percent by weight of aluminium of thickness 0.060 inches for 6 minutes at 250C, using an air pressure of 150 pounds per square inch.

The moulding was removed from the mould by cooling it around the peripheral flange and allowing it to fall under gravity onto the lower platen of the mould press. This caused some distortion of the domed portion of the moulding due to impact of the moulding on the lower platen.

This distortion was removed by inverting the hot (about 250C) moulding, placing it in another mould of complementary shape, and allowing it to resume its domed profile as defined by the mould surface. Cooling took place over approximately I hour.

I claim:

1. In a method of moulding superplastic zinc- /aluminium alloy sheet material containing from 15 to 40 percent by weight of aluminium comprising heating a blank of the sheet material to a temperature at which the material exhibits superplasticity, moulding the heated blank to conform to a mould surface of desired shape, and removing the moulded sheet from the mould, the improvement comprising holding the moulded sheet at a temperature of from 180 to 220C to remove undesired warping or distortions from the moulded sheet.

2. A method as claimed in claim 1 comprising carrying out the heat treatment of the moulded sheet for at least 10 minutes.

3. A method as claimed in claim 1 comprising cooling at least one of the surface of the moulded sheet adjacent the mould and the surface of the moulded sheet remote from the mould to facilitate removal of the moulded sheet from the mould.

4. A method as claimed in claim 3 comprising cooling the said at least one surface of the moulded sheet by air blowing.

5. A method as claimed in claim 3 comprising cooling said at least one surface of the moulded sheet by water spraying.

6. A method as claimed in claim 1 comprising supporting the moulded sheet during the heat treatment on a surface complementary to the surface of the sheet.

7. A method as claimed in claim 1 comprising moulding the heated blank of sheet material to conform to a moulded surface of desired shape by an applied fluid pressure.

8. A method as claimed in claim 1 comprising moulding the heated blank of sheet material to conform to a moulded surface of desired shape by an applied vacuum.

9. A method as claimed in claim 1 wherein the zinc- /aluminium alloy contains up to 0.5 percent by weight of copper or magnesium. 

2. A method as claimed in claim 1 comprising carrying out the heat treatment of the moulded sheet for at least 10 minutes.
 3. A method as claimed in claim 1 comprising cooling at least one of the surface of the moulded sheet adjacent the mould and the surface of the moulded sheet remote from the mould to facilitate removal of the moulded sheet from the mould.
 4. A method as claimed in claim 3 comprising cooling the said at least one surface of the moulded sheet by air blowing.
 5. A method as claimed in claim 3 comprising cooling said at least one surface of the moulded sheet by water spraying.
 6. A method as claimed in claim 1 comprising supporting the moulded sheet during the heat treatment on a surface complementary to the surface of the sheet.
 7. A method as claimed in claim 1 comprising moulding the heated blank of sheet material to conform to a moulded surface of desired shape by an applied fluid pressure.
 8. A method as claimed in claIm 1 comprising moulding the heated blank of sheet material to conform to a moulded surface of desired shape by an applied vacuum.
 9. A method as claimed in claim 1 wherein the zinc/aluminium alloy contains up to 0.5 percent by weight of copper or magnesium. 